In the prior art, buildings or vehicles, such as recreational vehicles, which are isolated from utility company power lines or, for other reasons, cannot be directly hooked up to utility company power lines use direct current power sources such as batteries for electrical power. For example, a typical recreational vehicle uses storage batteries to provide electrical power to any electrical devices when it is mobile. Such devices may include refrigerators, radios, televisions and microwave ovens. Direct current from the storage batteries is converted into alternating current through the use of an inverter.
When the electrical load presented by these devices is high, a large inverter may operate efficiently. However, when the load presented by the devices within the home or recreational vehicle drops, keeping a large inverter running requires an excessive amount of energy. For example, an idling large inverter may consume 12-15 watts in a typical recreational vehicle application. This excessive energy use prematurely drains storage batteries and provides no power other than to maintain the inverter in a ready state. In the prior art, attempts have been made to sense drops in such electrical loads and to respond to lower loads by removing or turning off the inverter if the load drops below a preselected threshold voltage. Unfortunately, it is very hard to determine the threshold at which the main inverter should be taken off line. These load sensing circuits are often known under the name of "sleep", "search", "idle current", or "idle mode" circuits. Also, there are certain electronic devices run by alternating current which present a very light load to the inverter. Such light loads as presented, for example, by digital clocks and electronic ballast lights are difficult to sense. Thus, when an inverter goes off line in response to a decrease in demand below a low threshold the use of such low power devices is interrupted until the inverter is turned on again by an increase in demand above the threshold. This is frustrating to the operator who must now reset these light load devices for the proper time or take other corrective action.
It is therefore a motivation of the present invention to maintain residual loads serviced by an inverter system while working at a higher efficiency of energy conversion than prior art devices.